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21.1 Genetic factors and hormones that determine gender
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Leydig's interstitial cells: function, mode of operation and hormonal secretion
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The interstitial cells (Leydig) probably have their origin in the mesonephros and develop outside the testicular cord in the testes. From the 7th week they secrete increasing amounts of testosterone. The maximum is attained in the course of the second trimester, when the final differentiation in the direction of the male phenotype takes place.
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As a result of stimulation with LH (luteinizing hormone) the interstitial cells (Leydig) secrete testosterone (biosynthesis of testosterone). The precursor molecule for testosterone is cholesterol. A complex cascade-like system of enzymes results in the biosynthesis of these steroids (scheme of the testicular steroids).Certain amounts of testosterone and androstendion penetrate into the Sertoli cells where, after a transformation into the more active form of the dihydrotestosterone, bind directly onto the corresponding receptors.
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The GnRH of the hypothalamus (Gonadotropin Releasing Hormone) affects the adenohypophysis and stimulates the release of LH (Luteinizing Hormone), which also has an influence on the testes.
The LH binds itself directly on specific receptors of the interstitial cells (Leydig) and causes the release of androgens (testosterone). For its transport in the blood, this fat-soluble hormone gets bound to a protein (BP = Binding Protein), thus getting into the whole organism and so to its target organs.
Important is the fact that testosterone also diffuses locally via the basal membrane outwardly and thus gets to the seminiferous tubules where it promotes the development of the germinal cells.
It is important to note that the synthesis of the specific receptor protein for the androgen (ABP) in the supporting cells (Sertoli) is coded by a gene on the X chromosome. A mutation of this gene is responsible for the insusceptibility to androgens (Feminizing testes: See pathology).
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In the target organ the testosterone diffuses through the cell membrane where it binds itself to the cytoplasmatic receptor proteins and is thus transported into the cell nucleus this way. Within the cell nucleus it binds to a nuclear receptor and this hormone-receptor-complex is ultimately able to bind itself to regions of the DNA, which regulates the transcription for a series of further proteins.
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Schematic diagram of the mode of operation of testosterone or the lipophilic hormones in general.
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| Depending on which type of target organ is involved, testosterone is either active itself (mesonephric duct [Wolff], epdidydimis, seminal vescicle, etc.) or works as a precursor hormone and only becomes active with its transformation through 5 a-reductase into dihydotestosterone (DHT) (outer genital organs, urogenital sinus). In addtion to its role in embryonic development testosterone is also important for the expression of the secondary characteristics of the masculine sex. |
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Fig. 1 - Phenotypic differentiation
through hormones in man |
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Urinary bladder
Deferent duct
Seminal vesicle
Prostate
Urethra
Epididymis
Testis
Pubic hair |
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Fig. 1
Influence on the formation and development of the secondary sex characteristics by testosterone (red) or dihydrotestosterone in a man (blue).
Note!
The color coding of this module does not apply in this drawing |
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